1. Field of the Invention
The present invention generally relates to mobile communication fields, and especially relates to a mobile communication terminal including a built-in radio tag.
2. Description of the Related Art
Recently, systems utilizing a radio tag have been attracting attention. Such a system includes a radio tag and a reader/writer device. The reader/writer device can read information from the radio tag and write information into the radio tag. The reader/writer device is referred to as an “interrogator”. The radio tag is referred to as an “RF tag”, “RFID”, “RFID tag”, “IC tag”, “electronic tag” and so on.
Data like identification (ID) or product number may be read and written via such a radio tag to manage and control product quality. Alternatively, information representing some values like tickets or points may be read, and therefore a variety of uses like electronic tickets, fare tickets or electronic money, not only product management, are expected. If such a radio tag is really built in a mobile communication terminal such as a cellular phone, the radio tag will become drastically more convenient, but the mobile communication terminal must have enough space to accommodate the radio tag.
However, such a radio tag requires a comparatively large space, and especially the antenna of the radio tag will occupy a large part of the volume of the mobile communication terminal. Japanese Patent Laid-Open No. 2003-37861 (Patent Document #1) discloses a technique where a contact-less IC card with a coiled antenna is mounted on a back surface of a battery of a mobile communication terminal to miniaturize the mobile communication terminal having the IC card.
Even if such a radio tag can be physically built in a mobile communication terminal, a further problem will remain. The frequency of the existing radio tag is 13.5 MHz, which is lower than the frequency of the mobile communication, and therefore significant limitations are placed on directions or distances where the radio tag can communicate. As for the mobile communication terminal as described in Patent Document #1, it is required to hold the IC card mounting surface right on the reader/writer device. If such optimum position is not obtained, radio tag communication becomes difficult. In the future, radio tags may utilize the UHF band frequencies (such as 953 MHz) so that communications directions and distances may be improved to some extent and therefore the above limitation on the communications directions and distances can be improved to some extent. However, the above problem cannot be completely solved.
On the other hand, as described in Patent Documents #2 and #3, one antenna is shared by a reader/writer device (communicating with a radio tag) and a mobile communication terminal (communicating a host computer) to reduce the size of the mobile communication terminal. In this kind of technique, it is premised that the impedance of the mobile communication circuit and the impedance of the reader/writer device circuit are the same.
FIG. 1 shows a Smith chart for illustrating impedance characteristics of a circuit or an antenna. Circles passing through a point P and including the whole line PR or a part thereof as diameters represent impedance locus (impedance characteristics) whose resistance components (real components) are the same. Arcs passing through and perpendicularly crossing all the above circles represent impedance locus (impedance characteristics) whose resistance components (complex components) are the same. Generally, a processing circuit in a mobile communication terminal has a characteristic impedance (complex component of zero and resistance component value of 50 Ω) of 50 Ω in the 800 MHz operating frequency band, and the impedance is indicated by a point Q. In Patent Documents #2 and #3, both the impedance Z1 of the mobile communication terminal and the impedance Z2 of the reader/writer device are placed (indicated by A) near the characteristic impedance, or an adjusting circuit is prepared in order to make those impedances Z1, Z2 be near the characteristic impedance Q.
On the other hand, the impedance of a radio tag processing circuit is much different from the characteristic impedance, and is indicated by point S. In this, both the impedance of the processing circuit in the mobile communication terminal and the impedance of the radio tag processing circuit are completely different from each other. Although an adjusting circuit may be introduced to make these impedances match each other, the introduction of such an adjusting circuit is against the requirement for miniaturizing the mobile communication terminal. Therefore, it is difficult to miniaturize a device having both functions as a mobile communication terminal and a radio tag using technologies disclosed in Patent Documents #2 and #3.
As explained above, when containing a radio tag in a mobile communication terminal, an additional antenna for the radio tag is needed in addition to an existing antenna for the mobile communication terminal. Mobile communication terminals desired to be miniaturized have no space to mount the additional antenna, which is a problem. Further, in general, housings of mobile communication terminals use many metallic parts such as a substrate, shield plates and so on, and therefore such mobile cellular phones with a built-in IC card as described in Patent Document #1 can only use their contact-less IC mounting surface for communicating with an external card reader/writer, which is inconvenient to users.                [Patent Document #1]        
Japanese Patent Laid-open No. 2003-37861                [Patent Document #2]        
Japanese Patent Laid-open No. 2001-307032                [Patent Document #3]        